1. Field of the Invention
This invention relates to a p-type Fe silicide thermoelectric conversion material for thermoelectric conversion devices, and more particularly to a material of this kind which is adapted for use in thermoelectric conversion devices for effecting thermoelectric power generation, thermoelectric cooling and heating, as well as for various sensing purposes.
2. Background Information
In general, a p-type thermoelectric conversion material, mating with an n-type thermoelectric conversion material, is used in thermoelectric conversion devices for thermoelectric power generation wherein thermal energy is directly converted into electric energy, ones for thermoelectric cooling or heating wherein cooling or heating is effected by causing electric current to flow therethrough, or other thermoelectric sensing devices.
Conventionally known p-type Fe silicide materials for thermoelectric conversion devices include one formed of a pure Fe silicide ("FeSi.sub.2 "), one proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 60-43881, which is formed of an Fe silicide in which Mn and Al are partly substituted respectively for part of Fe and part of Si (hereinafter called "the Mn-Al substitution type"), and one formed of an Fe silicide part of which is formed of a metallic phase (hereinafter called "the metallic phase type").
P-type thermoelectric conversion materials of this kind are generally required to have a large Seebeck coefficient S (.mu.V/.degree. C.) for obtaining an adequate thermoelectromotive force or satisfactory thermoelectric cooling effect, a small specific resistance .rho. for reducing generation of Joule heat, and a small thermal conductivity K for obtaining an adequate temperature difference across the material.
The maximum output Pmax of a p-type thermoelectric conversion material which is employed as a thermoelectric power generation element can be expressed by the following equation: ##EQU1## where S is the Seebeck coefficient, .rho. the specific resistance, and .DELTA.T the difference in temperature between two predetermined points of the p-type material. It is clear from the above equation that the maximum output Pmax of the p-type thermoelectric conversion material increases with an increase in an output factor represented by S.sup.2 /.rho. in the same equation.
The pure FeSi.sub.2 has a large Seebeck coefficient S, but it has a large specific resistance .rho., i.e. several .OMEGA./cm so that it is too small in the maximum output Pmax to be practically used as a thermoelectric power generation element.
The metallic phase type has a small specific resistance .rho., but the Seebeck coefficient S is as small as several .mu.V/.degree. C. As a consequence, the metallic phase type also has an insufficient maximum output Pmax and hence fails to exhibit improved thermoelectric power generation properties over the pure FeSi.sub.2.
On the other hand, the Mn-Al substitution type has a satisfactory Seebeck coefficient S, i.e. 150 to 350 .mu.V/.degree. C., and specific resistance .rho. of 5 to 10 m.OMEGA./cm, and can therefore form an excellent thermoelectric conversion element. However, the specific resistance .rho. of 5 to 10 m.OMEGA./cm is still large for the material to be used as a thermoelectric power generation element because the output factor S.sup.2 /.rho. is small and accordingly the maximum output pmax is insufficient.